When a cable develops a fault, an impedance discontinuity sometimes develops at that point in the cable. A technique, referred to as time domain reflectometry (TDR) is often used as one of the fault location methods. In this method, the TDR instrument is connected to the cable under test which applies a broadband, low voltage pulse to the cable. This pulse travels along the cable until it encounters an impedance discontinuity. When describing the phenomena of traveling waves in a cable system the cables are often specified in terms of their characteristic impedance, Z.sub.0, and their velocity of propagation, v.sub.0. If the fault's impedance discontinuity, Z.sub.1, is substantially different from Z.sub.0 then part of the pulse is reflected from the discontinuity and travels back to the instrument where it can be observed by the operator. This reflected pulse behaves much like an echo. The operator knows the time, T.sub.1, when the pulse was launched and can measure the arrival time, T.sub.2, of the reflected pulse. The location of the impedance discontinuity can be calculated as: ##EQU1## The factor of two appears because the pulse must travel to the impedance discontinuity and back. The amplitude of the reflected pulse depends on the magnitude of the impedance discontinuity. Assuming that the amplitude of the pulse applied to the cable is one volt and the impedance discontinuity, Z.sub.1, is a resistive shunt element, then the reflected pulse will have an amplitude equal to the reflection coefficient, .rho..sub.1, which is given by: ##EQU2## In practice impedance discontinuities resulting in reflection coefficients having magnitudes less than 0.1 are difficult to identify. Faults with reflection coefficients in the range of -1.0 to -0.1 are referred to as bolted faults and are usually detectable using TDR.
Locating faults in cables using TDR is often unsuccessful because an impedance discontinuity (short circuit) is only present while operating voltages are applied to the faulted cable. This type of fault is referred to as a high impedance fault and without operating voltages applied the cable appears functional. A method referred to as the arc reflection test (ART) circumvents this difficulty by simultaneously applying TDR pulses and operating level voltages. When operating level voltages are applied to the faulted cable an arc forms at the fault site. The arc has an extremely low impedance which causes the reflection coefficient to be nearly -1 at the fault site. While the fault is arcing TDR pulses are applied to the cable and the fault is easily identified because the reflection is strong. Often this technique is not successful with bolted faults because the change in the fault impedance (reflection coefficient) is small during the time that the arc is present and the operator cannot discern this change.
Both of the aforementioned techniques use TDR as a basis and TDR measurements are often difficult to interpret. Typical cables have other impedance discontinuities distributed along their length which have reflection coefficients large enough to mask the presence of the reflection from the fault. Unshielded multiconductor cables and direct buried cables with corroded neutral conductors are two notable examples where the other background reflections can potentially mask the desired reflection. Another important example is a branched cable system that has multiple cables tapped from it along its length.
A new TDR technique has recently become available and is referred to as differential TDR (DTDR). This technique allows the operator to remove nuisance reflections from the measurements. The basic idea is to use a TDR to measure the background scattering function of the cable and use it as a calibration signature of the cable to correct the measurements. The background scattering function is a signature of all the "background" discontinuities along the length of the cable under test which is usually referred to as clutter. The received, background scattering function, R.sub.bkgd (t), can be represented, using a first order model, as ##EQU3## where S(t) is the pulse that is applied to the cable, .rho..sub.i is the reflection coefficient of the i.sup.th impedance discontinuity, T.sub.i is the round-trip time delay to the i.sup.th impedance discontinuity and N is the number of significant reflectors along the cable. The DTDR technique is meant to be used in situations where a preexisting calibration for the cable is stored in an archive or where the operator has complete access to all parts of the system. Archives of the cable discontinuities often do not exist which limits the applicability of the archival technique. In the absence of an archive the DTDR technique finds its greatest use in tests where the operator determines the response of the system to a physical change made at some point in which there is interest. This physical change modifies the impedance at that point and thus the reflection coefficient. The operator then repeats the same measurement. The new measurement contains the same information as the background scattering function but also contains the reflection due to the physical change. The new received signal, R.sub.new (t), has the form ##EQU4## where .rho..sub.j is the reflection coefficient due to the physical change in the system and T.sub.j is the round-trip time delay to the point in the system where the change was made. The background scattering function, R.sub.bkgd (t), is subtracted from the new measurement, R.sub.new (t), which leaves as a residual the reflection from the impedance discontinuity of interest. The residual, R.sub.res (t), is given by EQU R.sub.res (t)=R.sub.new (t)-R.sub.bkgd (t)=.rho..sub.j S(t-T.sub.j)
which is the desired result. Knowledge of the "background" discontinuities enables one to use signal processing to remove the clutter masking the desired reflection. Under the proper conditions reflection coefficients with magnitudes of 0.0001 can be easily detected. The drawback to this technique is that it requires that the operator have access to the physical location in the system in which he is interested so that a physical change (change in impedance) can be made at that point.
Nature of the Present Invention
The new technique, Differential Arc Reflection Test (DART), combines two recognized fault location methods, TDR and ART, with the DTDR calibration technique to yield a nonambiguous measurement of the fault location, even in the presence of heavy clutter. The technique is similar to DTDR except that the impedance discontinuity at the fault site is changed by inducing an arc at the fault site by applying the necessary voltage to the cable. Normal usage of DTDR would require that the operator have access to the fault site to make an impedance change which implies that the location of the fault is already known. The DART technique induces the impedance change from the terminal of the cable.
As in the DTDR method the background scattering function of the faulted cable is measured without normal operating voltage levels applied to the cable. The received signal, R.sub.bkgd (t), is represented, using a first order model, as ##EQU5## where S(t) is the pulse that is launched into the cable, .rho..sub.i is the reflection coefficient of the i.sup.th impedance discontinuity, T.sub.i is the round-trip time delay to the i.sup.th impedance discontinuity and N is the number of significant reflectors along the cable up to the point of the fault. Only reflectors up to the location of the fault are used in this description because they are the only reflectors that influence the results.
A voltage sufficient to induce arcing at the fault is applied to the cable under test. Simultaneously to the voltage application TDR pulses are applied to the cable. The new measurement contains the same information as the background scattering function but also contains the reflection due to the low impedance at the fault. The new received signal, R.sub.new (t), has the form ##EQU6## where the reflection coefficient is assumed to be -1 at the fault site and T.sub.j is the round-trip time delay to the fault site.
The background scattering function, R.sub.bkgd (t), is subtracted from the new measurement, R.sub.new (t), which leaves as a residual the reflection from the fault site. The residual, R.sub.res (t), is given by EQU R.sub.res (t)=R.sub.new (t)-R.sub.bkgd (t)=-S(t-T.sub.j) (2)
which is the desired result. The delay time, T.sub.j, is then converted into the fault location by applying Eq. 1 as previously discussed. Knowledge of the background discontinuities enables one to use signal processing to remove the clutter masking the reflection from the fault.
The above analysis applies only to reflections returning to the instrument up to and including time T.sub.j. The residual signal resulting from the DART technique will be zero up to the time T.sub.j and will contain undetermined returns after that time. There are two reasons for this behavior of the residual. When the arc forms very little pulse energy travels past the fault impedance and the received signal does not contain any of the background scattering function past the fault. Also, other, new, multiple reflections result from the introduction of the new impedance discontinuity (the arc) but these reflections arrive at the instrument at a later time than T.sub.j. These extraneous returns do not influence the results since one is only interested in the return occurring at T.sub.j ; the return at T.sub.j is completely unmasked from clutter. For these reasons one is only interested in the first significant return in the residual, R.sub.res (t).
This technique also applies to bolted faults in cables. If sufficient energy is applied to the cable under test, the current through the fault begins to heat the fault impedance changing its impedance slightly. This small change in impedance is easily detectable using DART with the amplitude of the result equaling the difference in the impedance before and during the arc. The residual, R.sub.res (t), will have the form EQU R.sub.res (t)=-.DELTA..rho..sub.j S(t-T.sub.j)
where .DELTA..rho..sub.j is the change in the reflection coefficient at the fault site.
The Invention
More specifically the method of the present invention relates to determining the location of a fault in electrical conductors from a terminal position which may be remote from the fault using at least a broadband pulse generator to generate a pulse at the terminal position, a voltage source at the terminal position sufficient to induce a change of impedance at the fault site and time measuring means also at the terminal position. At least the following steps are involved: generating a first pulse at the terminal position to be propagated down the conductor and reflected back to the terminal position from various impedance discontinuities including the impedance discontinuity at the fault, if present; recording the pattern of the reflections from points along the conductor with the fault resulting from the first pulse; applying sufficient voltage to induce a change in impedance at the fault and simultaneously generating a second pulse, similar to the first, at the terminal position to be propagated down the conductor and reflected back to the terminal position from the various impedance discontinuities up to the fault and from the modified impedance discontinuity at the fault; subtracting the reflections from the first pulse from the reflections of the second pulse and determining from the remaining major reflections, the location of the fault. The method is preferably practiced using the formula in Eq. 2 as previously described.
The fault location apparatus of the present invention includes at least a broadband pulse generator for generating a pulse at a terminal of an electrical conductor to be tested and a voltage source capable of producing a momentary impedance change at the fault site from the terminal position. A filter is used to separate the voltage inducing the change in impedance at the fault to be located from the broadband time domain reflectometry pulse. Means are provided to record the pulse reflections digitally from points along the electrical conductor and to eliminate by subtraction pulse reflections other than the different pulses generated by the change in impedance at the fault to be located. Means are provided to calculate the time between the pulse initiated by the pulse generator and a pulse to be received back from the fault.